conditions and physical changes which have affected the modification and
dispersal of organisms, and have thus brought about their actual
distribution on {126} the surface of the earth. These studies will, it is
believed, place us in a condition to solve most of the problems presented
by the distribution of animals and plants, whenever the necessary facts,
both as to their distribution and their affinities, are sufficiently well
known; and we now proceed to apply the principles we have established to
the interpretation of the phenomena presented by some of the more important
and best known of the islands of our globe, limiting ourselves to these for
reasons which have been already sufficiently explained in our preface.
* * * * *
PART II
_INSULAR FAUNAS AND FLORAS_
{241}
CHAPTER XI
THE CLASSIFICATION OF ISLANDS
Importance of Islands in the Study of the Distribution of
Organisms--Classification of Islands with Reference to
Distribution--Continental Islands--Oceanic Islands.
In the preceding chapters, forming the first part of our work, we have
discussed, more or less fully, the general features presented by animal
distribution, as well as the various physical and biological changes which
have been the most important agents in bringing about the present condition
of the organic world.
We now proceed to apply these principles to the solution of the numerous
problems presented by the distribution of animals; and in order to limit
the field of our inquiry, and at the same time to deal only with such facts
as may be rendered intelligible and interesting to those readers who have
not much acquaintance with the details of natural history, we propose to
consider only such phenomena as are presented by the islands of the globe.
_Importance of Islands in the Study of the Distribution of
Organisms._--Islands possess many advantages for the study of the laws and
phenomena of distribution. As compared with continents they have a
restricted area and definite boundaries, and in most cases their
geographical and biological limits coincide. The number of species and of
genera they contain is always much smaller than in the {242} case of
continents, and their peculiar species and groups are usually well defined
and strictly limited in range. Again, their relations with other lands are
often direct and simple, and even when more complex are far easier to
comprehend than those of continents; and they exhibit besides certain
influences on the forms of life and certain peculiarities in their
distribution which continents do not present, and whose study offers many
points of interest.
In islands we have the facts of distribution presented to us, sometimes in
their simplest forms, in other cases becoming gradually more and more
complex; and we are therefore able to proceed step by step in the solution
of the problems they present. But as in studying these problems we have
necessarily to take into account the relations of the insular and
continental faunas, we also get some knowledge of the latter, and acquire
besides so much command over the general principles which underlie all
problems of distribution, that it is not too much to say that when we have
mastered the difficulties presented by the peculiarities of island life we
shall find it comparatively easy to deal with the more complex and less
clearly defined problems of continental distribution.
_Classification of Islands with Reference to Distribution._--Islands have
had two distinct modes of origin--they have either been separated from
continents of which they are but detached fragments, or they have
originated in the ocean and have never formed part of a continent or any
large mass of land. This difference of origin is fundamental, and leads to
a most important difference in their animal inhabitants; and we may
therefore first distinguish the two classes--oceanic and continental
islands.
Mr. Darwin appears to have been the first writer who called attention to
the number and importance, both from a geological and biological point of
view, of oceanic islands. He showed that with very few exceptions all the
remoter islands of the great oceans were of volcanic or coralline
formation, and that none of them contained indigenous mammalia or amphibia.
He also showed the connection of these two phenomena, and maintained that
none of the islands so characterised had ever formed {243} part of a
continent. This was quite opposed to the opinions of the scientific men of
the day, who almost all held the idea of continental extensions, and of
oceanic islands being their fragments, and it was long before Mr. Darwin's
views obtained general acceptance. Even now the belief still lingers; and
we continually hear of old Atlantic or Pacific continents, of "Atlantis" or
"Lemuria," of which hypothetical lands many existing islands, although
wholly volcanic, are thought to be the remnants. We have already seen that
Darwin connected the peculiar geological structure of oceanic islands with
the permanence of the great oceans which contain them, and we have shown
that several distinct lines of evidence all point to the same conclusion.
We may therefore define oceanic islands, as follows:--Islands of volcanic
or coralline formation, usually far from continents and always separated
from them by very deep sea, entirely without indigenous land mammalia or
amphibia, but with a fair number of birds and insects, and usually with
some reptiles. This definition will exclude only two islands which have
been sometimes classed as oceanic--New Zealand and the Seychelles.
Rodriguez, which was once thought to be another exception, has been shown
by the explorations during the Transit of Venus Expedition to be
essentially volcanic, with some upraised coralline limestone.
_Continental Islands._--Continental islands are always more varied in their
geological formation, containing both ancient and recent stratified rocks.
They are rarely very remote from a continent, and they always contain some
land mammals and amphibia, as well as representatives of the other classes
and orders in considerable variety. They may, however, be divided into two
well-marked groups--ancient and recent continental islands--the characters
of which may be easily defined.
Recent continental islands are always situated on submerged banks
connecting them with a continent, and the depth of the intervening sea
rarely exceeds 100 fathoms. They resemble the continent in their geological
structure, while their animal and vegetable productions are either almost
identical with those of the continent, or if {244} otherwise, the
difference consists in the presence of closely allied species of the same
types, with occasionally a very few peculiar genera. They possess in fact
all the characteristics of a portion of the continent, separated from it at
a recent geological period.
Ancient continental islands differ greatly from the preceding in many
respects. They are not united to the adjacent continent by a shallow bank,
but are usually separated from it by a depth of sea of several hundreds to
more than a thousand fathoms. In geological structure they agree generally
with the more recent islands; like them they possess mammalia and amphibia,
usually in considerable abundance, as well as all other classes of animals;
but these are highly peculiar, almost all being distinct species, and many
forming distinct and peculiar genera or families. They are also well
characterised by the fragmentary nature of their fauna, many of the most
characteristic continental orders or families being quite unrepresented,
while some of their animals are allied, not to such forms as inhabit the
adjacent continent, but to others found only in remote parts of the world.
This very remarkable set of characters marks off the islands which exhibit
them as a distinct class, which often present the greatest anomalies and
most difficult problems to the student of distribution.
_Oceanic Islands._--The total absence of warm-blooded terrestrial animals
in an island otherwise well suited to maintain them, is held to prove that
such island is no mere fragment of any existing or submerged continent, but
one that has been actually produced in mid-ocean. It is true that if a
continental island were to be completely submerged for a single day and
then again elevated, its higher terrestrial animals would be all destroyed,
and if it were situated at a considerable distance from land it would be
reduced to the same zoological condition as an oceanic island. But such a
complete submergence and re-elevation appears never to have taken place,
for there is no single island on the globe which has the physical and
geological features of a continental, combined with the zoological features
of an oceanic island. It is true that some of the coral-islands may be
formed upon submerged lands {245} of a continental character, but we have
no proof of this; and even if it were so, the existing islands are to all
intents and purposes oceanic.
We will now pass on to a consideration of some of the more interesting
examples of these three classes, beginning with oceanic islands.
All the animals which now inhabit such oceanic islands must either
themselves have reached them by crossing the ocean, or be the descendants
of ancestors who did so. Let us then see what are, in fact, the animal and
vegetable inhabitants of these islands, and how far their presence can be
accounted for. We will begin with the Azores, or Western Islands, because
they have been thoroughly well explored by naturalists, and in their
peculiarities afford us an important clue to some of the most efficient
means of distribution among several classes of animals.
* * * * *
{246}
CHAPTER XII
OCEANIC ISLANDS:--THE AZORES AND BERMUDA
THE AZORES, OR WESTERN ISLANDS
Position and Physical Features--Chief Zoological Features of the
Azores--Birds--Origin of the Azorean Bird Fauna--Insects of the
Azores--Land-Shells of the Azores--The Flora of the Azores--The
Dispersal of Seeds--Birds as Seed-Carriers--Facilities for Dispersal of
Azorean Plants--Important Deduction from the Peculiarities of the
Azorean Fauna and Flora.
BERMUDA
Position and Physical Features--The Red Clay of Bermuda--Zoology of
Bermuda--Birds of Bermuda--Comparison of the Bird Faunas of Bermuda and
the Azores--Insects of Bermuda--Land Mollusca--Flora of
Bermuda--Concluding Remarks on the Azores and Bermuda.
We will commence our investigation into the phenomena presented by oceanic
islands, with two groups of the North Atlantic, in which the facts are of a
comparatively simple nature and such as to afford us a valuable clue to a
solution of the more difficult problems we shall have to deal with further
on. The Azores and Bermuda offer great contrasts in physical features, but
striking similarities in geographical position. The one is volcanic, the
other coralline; but both are surrounded by a wide expanse of ocean of
enormous depth, the one being about as far from Europe as the other is from
America. Both are situated in the {247} temperate zone, and they differ
less than six degrees in latitude, yet the vegetation of the one is wholly
temperate, while that of the other is almost tropical. The productions of
the one are related to Europe, as those of the other are to America, but
they present instructive differences; and both afford evidence of the
highest value as to the means of dispersal of various groups of organisms
across a wide expanse of ocean.
THE AZORES, OR WESTERN ISLANDS.
These islands, nine in number, form a widely scattered group, situated
between 37deg and 39deg 40' N. Lat. and stretching in a south-east and
north-west direction over a distance of nearly 400 miles. The largest of
the islands, San Miguel, is about forty miles long, and is one of the
nearest to Europe, being rather under 900 miles from the coast of Portugal,
from which it is separated by an ocean 2,500 fathoms deep. The depth
between the islands does not seem to be known, but the 1,000 fathom line
encloses the whole group pretty closely, while a depth of about 1,800
fathoms is reached within 300 miles in all directions. These great depths
render it in the highest degree improbable that the Azores have ever been
united with the European continent; while their being wholly volcanic is
equally opposed to the view of their having formed part of an extensive
Atlantis including Madeira and the Canaries. The only exception to their
volcanic structure is the occurrence in one small island only (Santa Maria)
of some marine deposits of Upper Miocene age--a fact which proves some
alterations of level, and perhaps a greater extension of this island at
some former period, but in no way indicates a former union of the islands,
or any greater extension of the whole group. It proves, however, that the
group is of considerable antiquity, since it must date back to Miocene
times; and this fact may be of importance in considering the origin and
peculiar features of the fauna and flora. It thus appears that in all
physical features the Azores correspond strictly with our physical
definition of "oceanic islands," while their great distance {248} from any
other land, and the depth of the ocean around them, make them typical
examples of the class. We should therefore expect them to be equally
typical in their fauna and flora; and this is the case as regards the most
important characteristics, although in some points of detail they present
exceptional phenomena.
[Illustration: OUTLINE MAP OF THE AZORES.]
NOTE.--
The light tint shows where the sea is less than 1,000 fathoms deep.
The dark tint " " " more than 1,000 fathoms deep.
The figures show depths in fathoms.
_Chief Zoological Features of the Azores._[50]--The great feature of
oceanic islands--the absence of all indigenous land-mammalia and
amphibia--is well shown in this {249} group; and it is even carried
further, so as to include all terrestrial vertebrata, there being no snake,
lizard, frog, or fresh-water fish, although the islands are sufficiently
extensive, possess a mild and equable climate, and are in every way adapted
to support all these groups. On the other hand, flying creatures, as birds
and insects, are abundant; and there is also one flying mammal--a small
European bat. It is true that rabbits, weasels, rats and mice, and a small
lizard peculiar to Madeira and Teneriffe, are now found wild in the Azores,
but there is good reason to believe that these have all been introduced by
human agency. The same may be said of the gold-fish and eels now found in
some of the lakes, there being not a single fresh-water fish which is truly
indigenous to the islands. When we consider that the nearest part of the
group is about 900 miles from Portugal, and more than 550 miles from
Madeira, it is not surprising that none of these terrestrial animals can
have passed over such a wide expanse of ocean unassisted by man.
Let us now see what animals are believed to have reached the group by
natural means, and thus constitute its indigenous fauna. These consist of
birds, insects, and land-shells, each of which must be considered
separately.
_Birds._--Fifty-three species of birds have been observed at the Azores,
but the larger proportion (thirty-one) are either aquatic or waders--birds
of great powers of flight, whose presence in the remotest islands is by no
means remarkable. Of these two groups twenty are residents, breeding in the
islands, while eleven are stragglers only visiting the islands
occasionally, and all are common European species. The land-birds,
twenty-two in number, are more interesting, four only being stragglers,
while eighteen are permanent residents. The following is a list of these
resident land-birds:--
1. Common Buzzard (_Buteo vulgaris_)
2. Long-eared Owl (_Asio otus_)
3. Barn Owl (_Strix flammea_)
4. Blackbird (_Turdus merula_)
5. Robin (_Erythacus rubecula_)
6. Blackcap (_Sylvia atricapilla_)
{250}
7. Gold-crest (_Regulus cristatus_)
8. Wheatear (_Saxicola oenanthe_)
9. Grey Wagtail (_Motacilla sulphurea_)
10. Atlantic Chaffinch (_Fringilla tintillon_)
11. Azorean Bullfinch (_Pyrrhula murina_)
12. Canary (_Serinus canarius_)
13. Common Starling (_Sturnus vulgaris_)
14. Lesser Spotted Woodpecker (_Dryobates minor_)
15. Wood-pigeon (_Columba palumbus_)
16. Rock Dove (_Columba livia_)
17. Red-legged Partridge (_Caccabis rufa_)
18. Common Quail (_Coturnix communis_)
All the above-named birds are common in Europe and North Africa except
three--the Atlantic chaffinch and the canary which inhabit Madeira and the
Canary Islands, and the Azorean bullfinch, which is peculiar to the islands
we are considering.
_Origin of the Azorean Bird-fauna._--The questions we have now before us
are--how did these eighteen species of birds first reach the Azores, and
how are we to explain the presence of a single peculiar species while all
the rest are identical with European birds? In order to answer them, let us
first see what stragglers now actually visit the Azores from the nearest
continents. The four species given in Mr. Godman's list are the kestrel,
the oriole, the snow-bunting, and the hoopoe; but he also tells us that
there are certainly others, and adds: "Scarcely a storm occurs in spring or
autumn without bringing one or more species foreign to the islands; and I
have frequently been told that swallows, larks, grebes, and other species
not referred to here, are not uncommonly seen at those seasons of the
year."
We have, therefore, every reason to believe that the birds which are now
residents originated as stragglers, which occasionally found a haven in
these remote islands when driven out to sea by storms. Some of them, no
doubt, still often arrive from the continent, but these cannot easily be
distinguished as new arrivals among those which are permanent inhabitants.
Many facts mentioned by Mr. Godman show that this is the case. A barn-owl,
much exhausted, flew on board a whaling-ship when 500 miles S.W. of the
Azores; and even if it had come from {251} Madeira it must have travelled
quite as far as from Portugal to the islands. Mr. Godman also shot a single
specimen of the wheatear in Flores after a strong gale of wind, and as no
one on the island knew the bird, it was almost certainly a recent arrival.
Subsequently a few were found breeding in the old crater of Corvo, a small
adjacent island; and as the species is not found in any other island of the
group, we may infer that this bird is a recent immigrant in process of
establishing itself.
Another fact which is almost conclusive in favour of the bird-population
having arrived as stragglers is, that they are most abundant in the islands
nearest to Europe and Africa. The Azores consist of three divisions--an
eastern, consisting of two islands, St. Michael's and St. Mary's; a central
of five, Terceira, Graciosa, St. George's, Pico, and Fayal; and a western
of two, Flores and Corvo. Now had the whole group once been united to the
continent, or even formed parts of one extensive Atlantic island, we should
certainly expect the central group, which is more compact and has a much
larger area than all the rest, to have the greatest number and variety of
birds. But the fact that birds are most numerous in the eastern group, and
diminish as we go westward, is entirely opposed to this theory, while it is
strictly in accordance with the view that they are all stragglers from
Europe, Africa, or the other Atlantic islands. Omitting oceanic wanderers,
and including all birds which have probably arrived involuntarily, the
numbers are found to be forty species in the eastern group, thirty-six in
the central, and twenty-nine in the western.
To account for the presence of one peculiar species--the bullfinch (which,
however, does not differ from the common European bullfinch more than do
some of the varieties of North American birds from their type-species) is
not difficult; the wonder rather being that there are not more peculiar
forms. In our third chapter we have seen how great is the amount of
individual variation in birds, and how readily local varieties become
established wherever the physical conditions are sufficiently distinct. Now
we can hardly have a greater difference of conditions {252} than between
the continent of Europe or North Africa, and a group of rocky islands in
mid-Atlantic, situated in the full course of the Gulf Stream and with an
excessively mild though stormy climate. We have every reason to believe
that special modifications would soon become established in any animals
completely isolated under such conditions. But they are not, as a rule,
thus completely isolated, because, as we have seen, stragglers arrive at
short intervals; and these, mixing with the residents, keep up the purity
of the breed. It follows, that only those species which reach the Azores at
very remote intervals will be likely to acquire well-marked distinctive
characters; and this appears to have happened with the bullfinch alone, a
bird which does not migrate, and is therefore less likely to be blown out
to sea, more especially as it inhabits woody districts. A few other Azorean
birds, however, exhibit slight differences from their European allies.
There is another reason for the very slight amount of peculiarity presented
by the fauna of the Azores as compared with many other oceanic islands,
dependent on its comparatively recent origin. The islands themselves may be
of considerable antiquity, since a few small deposits, believed to be of
Miocene age, have been found on them, but there can be little doubt that
their present fauna, at all events as concerns the birds, had its origin
since the date of the last glacial epoch. Even now icebergs reach the
latitude of the Azores but a little to the west of them; and when we
consider the proofs of extensive ice-action in North America and Europe, we
can hardly doubt that these islands were at that time surrounded with
pack-ice, while their own mountains, reaching 7,600 feet high in Pico,
would almost certainly have been covered with perpetual snow and have sent
down glaciers to the sea. They might then have had a climate almost as bad
as that now endured by the Prince Edward Islands in the southern
hemisphere, nearly ten degrees farther from the equator, where there are no
land-birds whatever, although the distance from Africa is not much greater
than that of the Azores from Europe, while the vegetation is limited to a
few alpine plants and mosses. This recent origin of the {253} birds
accounts in a great measure for their identity with those of Europe,
because, whatever change has occurred must have been effected in the
islands themselves, and in a time limited to that which has elapsed since
the glacial epoch passed away.
_Insects of the Azores._--Having thus found no difficulty in accounting for
the peculiarities presented by the birds of these islands, we have only to
see how far the same general principles will apply to the insects and
land-shells. The butterflies, moths, and hymenoptera, are few in number,
and almost all seem to be common European species, whose presence is
explained by the same causes as those which have introduced the birds.
Beetles, however, are more numerous, and have been better studied, and
these present some features of interest. The total number of species yet
known is 212, of which 175 are European; but out of these 101 are believed
to have been introduced by human agency, leaving seventy-four really
indigenous. Twenty-three of these indigenous species are not found in any
of the other Atlantic islands, showing that they have been introduced
directly from Europe by causes which have acted more powerfully here than
farther south. Besides these there are thirty-six species not found in
Europe, of which nineteen are natives of Madeira or the Canaries, three are
American, and fourteen are altogether peculiar to the Azores. These latter
are mostly allied to species found in Europe or in the other Atlantic
islands, while one is allied to an American species, and two are so
distinct as to constitute new genera. The following list of these peculiar
species will be interesting:--
CARABIDAE.
_Anchomenus aptinoides_ Allied to a species from the Canaries.
_Bembidium hesperus_ Allied to the European _B. laetum_.
DYTISCIDAE.
_Agabus godmanni_ Allied to the European _A. dispar_.
COLYDIIDAE.
_Tarphius wollastoni_ A genus almost peculiar to the Atlantic
islands.
{254}
ELATERIDAE.
_Heteroderes azoricus_ Allied to a Brazilian species.
_Elastrus dolosus_ Belongs to a peculiar Madagascar genus!
MELYRIDAE.
_Attalus miniaticollis_ Allied to a Canarian species.
RHYNCOPHORA.
_Phlaeophagus variabilis_ Allied to European and Atlantic species.
_Acalles droueti_ A Mediterranean and Atlantic genus.
_Laparocerus azoricus_ Allied to Madeiran species.
_Asynonychun godmansi_ A peculiar genus, allied to _Brachyderes_, of
the south of Europe.
_Neocnemis occidentalis_ A peculiar genus, allied to the European
genus _Strophosomus_.
HETEROMERA.
_Helops azoricus_ Allied to _H. vulcanus_ of Madeira.
STAPHYLINIDAE.
_Xenomma melanocephala_ Allied to _X. filiforme_ from the Canaries.
This greater amount of speciality in the beetles than in the birds may be
due to two causes. In the first place many of these small insects have no
doubt survived the glacial epoch, and may, in that case, represent very
ancient forms which have become extinct in their native country; and in the
second place, insects have many more chances of reaching remote islands
than birds, for not only may they be carried by gales of wind, but
sometimes, in the egg or larva state or even as perfect insects, they may
be drifted safely for weeks over the ocean, buried in the light stems of
plants or in the solid wood of trees in which many of them undergo their
transformations. Thus we may explain the presence of three common South
American species (two elaters and a longicorn), all wood-eaters, and
therefore liable to be occasionally brought in floating timber by the Gulf
Stream. But insects are also immensely more numerous in species than are
land-birds, and their transmission would be in most cases quite
involuntary, and not dependent on their own powers of flight as with birds;
and thus the chances against the same species being frequently carried to
the same island would be considerable. If we add to this the dependence of
so {255} many insects on local conditions of climate and vegetation, and
their liability to be destroyed by insectivorous birds, we shall see that,
although there may be a greater probability of insects as a whole reaching
the islands, the chance against any particular species arriving there, or
against the same species arriving frequently, is much greater than in the
case of birds. The result is, that (as compared with Britain for example)
the birds are, proportionately, much more numerous than the beetles, while
the peculiar species of beetles are much more numerous than among birds,
both facts being quite in accordance with what we know of the habits of the
two groups. We may also remark, that the small size and obscure characters
of many of the beetles renders it probable that species now supposed to be
peculiar, really inhabit some parts of Europe or North Africa.
It is interesting to note that the two families which are pre-eminently
wood, root, or seed eaters, are those which present the greatest amount of
speciality. The two Elateridae alone exhibit remote affinities, the one
with a Brazilian the other with a Madagascar group; while the only peculiar
genera belong to the Rhyncophora, but are allied to European forms. These
last almost certainly form a portion of the more ancient fauna of the
islands which migrated to them in pre-glacial times, while the Brazilian
elater appears to be the solitary example of a living insect brought by the
Gulf Stream to these remote shores. The elater, having its nearest living
ally in Madagascar (_Elastrus dolosus_), cannot be held to indicate any
independent communication between these distant islands; but is more
probably a relic of a once more widespread type which has only been able to
maintain itself in these localities. Mr. Crotch states that there are some
_species_ of beetles common to Madagascar and the Canary Islands, while
there are several _genera_, common to Madagascar and South America, and
some to Madagascar and Australia. The clue to these apparent anomalies is
found in other genera being common to Madagascar, Africa, and South
America, while others are Asiatic or Australian. Madagascar, in fact, has
insect relations with every part of {256} the globe, and the only rational
explanation of such facts is, that they are indications of very ancient and
once widespread groups, maintaining themselves only in a few widely
separated portions of what was at one time or another the area of their
distribution.
_Land-shells of the Azores._--Like the insects and birds, the land-shells
of these islands have a generally European aspect, but with a larger
proportion of peculiar species. This was to be expected, because the means
by which molluscs are carried over the sea are far less numerous and varied
than in the case of insects;[51] and we may therefore conclude that their
introduction is a very rare event, and that a species once arrived remains
for long periods undisturbed by new arrivals, and is therefore more likely
to become modified by the new conditions, and then fixed as a distinct
type. Out of the sixty-nine known species, thirty-seven are common to
Europe or the other Atlantic islands, while thirty-two are peculiar, though
almost all are distinctly allied to European types. The majority of these
shells, especially the peculiar forms, are very small, and many of them may
date back to beyond the glacial epoch. The eggs of these would be
exceedingly minute, and might occasionally be carried on leaves or other
materials during gales of exceptional violence and duration, while others
might be conveyed with the earth that often sticks to the feet of birds.
There are also, probably, other unknown means of conveyance; but however
this may be, the general character of the land-molluscs is such as to
confirm the conclusions we have arrived at from a study of the birds and
insects,--that these islands have never been connected with a continent,
and have been peopled with living things by such forms only as in some way
or other have been able to reach them across many hundred miles of ocean.
_The Flora of the Azores._--The flowering-plants of the Azores have been
studied by one of our first botanists, Mr. H. C. Watson, who has himself
visited the islands and made extensive collections; and he has given a
complete catalogue of the species in Mr. Godman's volume. As our {257}
object in the present work is to trace the past history of the more
important islands by means of the forms of life that inhabit them, and as
for this purpose plants are sometimes of more value than any class of
animals, it will be well to take advantage of the valuable materials here
available, in order to ascertain how far the evidence derived from the two
organic kingdoms agrees in character; and also to obtain some general
results which may be of service in our discussion of more difficult and
more complex problems.
There are in the Azores 480 known species of flowering-plants and ferns, of
which no less than 440 are found also in Europe, Madeira, or the Canary
Islands; while forty are peculiar to the Azores, but are more or less
closely allied to European species. As botanists are no less prone than
zoologists to invoke former land-connections and continental extensions to
account for the wide dispersal of objects of their study, it will be well
to examine somewhat closely what these facts really imply.
_The Dispersal of Seeds._--The seeds of plants are liable to be dispersed
by a greater variety of agents than any other organisms, while their
tenacity of life, under varying conditions of heat and cold, drought and
moisture, is also exceptionally great. They have also an advantage, in that
the great majority of flowering plants have the sexes united in the same
individual, so that a single seed in a state fit to germinate may easily
stock a whole island. The dispersal of seeds has been studied by Sir Joseph
Hooker, Mr. Darwin, and many other writers, who have made it sufficiently
clear that they are in many cases liable to be carried enormous distances.
An immense number are specially adapted to be carried by the wind, through
the possession of down or hairs, or membranous wings or processes; while
others are so minute, and produced in such profusion, that it is difficult
to place a limit to the distance they might be carried by gales of wind or
hurricanes. Another class of somewhat heavier seeds or dry fruits are
capable of being exposed for a long time to sea-water without injury. Mr.
Darwin made many experiments on this point, and he found that many seeds,
especially of Atriplex, {258} Beta, oats, Capsicum, and the potato, grew
after 100 days' immersion, while a large number survived fifty days. But he
also found that most of them sink after a few days' immersion, and this
would certainly prevent them being floated to very great distances. It is
very possible, however, that dried branches or flower-heads containing
seeds would float longer, while it is quite certain that many tropical
seeds do float for enormous distances, as witness the double cocoa-nuts
which cross the Indian ocean from the Seychelle Islands to the coast of
Sumatra, and the West Indian beans which frequently reach the west coast of
Scotland. There is therefore ample evidence of the possibility of seeds
being conveyed across the sea for great distances by winds and surface
currents.[52]
_Birds as Seed-carriers._--The great variety of fruits that are eaten by
birds afford a means of plant-dispersal in the fact that seeds often pass
through the bodies of birds in a state well-fitted for germination; and
such seeds may occasionally be carried long distances by this means. Of the
twenty-two land-birds found in the Azores, half are, more or less,
fruit-eaters, and these may have been the means of introducing many plants
into the islands.
Birds also frequently have small portions of earth on their feet; and Mr.
Darwin has shown by actual experiment that almost all such earth contains
seeds. Thus in {259} nine grains of earth on the leg of a woodcock a seed
of the toad-rush was found which germinated; while a wounded red-legged
partridge had a ball of earth weighing six and a half ounces adhering to
its leg, and from this earth Mr. Darwin raised no less than eighty-two
separate plants of about five distinct species. Still more remarkable was
the experiment with six and three-quarter ounces of mud from the edge of a
little pond, which, carefully treated under glass, produced 537 distinct
plants! This is equal to a seed for every six grains of mud, and when we
consider how many birds frequent the edges of ponds in search of food, or
come there to drink, it is evident that great numbers of seeds may be
dispersed by this means.
Many seeds have hispid awns, hooks, or prickles which readily attach them
to the feathers of birds, and a great number of aquatic birds nest inland
on the ground; and as these are pre-eminently wanderers, they must often
aid in the dispersal of such plants.[53]
{260}
_Facilities for Dispersal of Azorean Plants._--Now in the course of very
long periods of time the various causes here enumerated would be sufficient
to stock the remotest islands with vegetation, and a considerable part of
the Azorean flora appears well adapted to be so conveyed. Of the 439
flowering-plants in Mr. Watson's list, I find that about forty-five belong
to genera that have either pappus or winged seeds; sixty-five to such as
have very minute seeds; thirty have fleshy fruits such as are greedily
eaten by birds; several have hispid seeds; and eighty-four are glumaceous
plants, which are all probably well-adapted for being carried partly by
winds and partly by currents, as well as by some of the other causes
mentioned. On the other hand we have a very suggestive fact in the absence
from the Azores of most of the trees and shrubs with large and heavy
fruits, however common they may be in Europe. Such are oaks, chestnuts,
hazels, apples, beeches, alders, and firs; while the only trees or large
shrubs are the Portugal laurel, myrtle, laurestinus, elder, _Laurus
canariensis_, _Myrica faya_, and a doubtfully peculiar juniper--all small
berry-bearers, and therefore likely to have been conveyed by one or other
of the modes suggested above.
There can be little doubt that the truly indigenous flora of the islands is
far more scanty than the number of plants recorded would imply, because a
large but unknown proportion of the species are certainly importations,
voluntary or involuntary, by man. As, however, the general character of the
whole flora is that of the south-western peninsula of Europe, and as most
of the introduced plants have come from the same country, it is almost
impossible now to separate them, and Mr. Watson has not attempted to do so.
The whole flora contains representatives of eighty natural orders and 250
genera: and even if we suppose that one-half the species only are truly
indigenous, {261} there will still remain a wonderfully rich and varied
flora to have been carried, by the various natural means above indicated,
over 900 miles of ocean, more especially as the large proportion of species
identical with those of Europe shows that their introduction has been
comparatively recent, and that it is, probably (as in the case of the
birds) still going on. We may therefore feel sure that we have here by no
means reached the limit of distance to which plants can be conveyed by
natural means across the ocean; and this conclusion will be of great value
to us in investigating other cases where the evidence at our command is
less complete, and the indications of origin more obscure or conflicting.
Of the forty species which are considered to be peculiar to the islands,
all are allied to European plants except six, whose nearest affinities are
in the Canaries or Madeira. Two of the Compositae are considered to be
distinct genera, but in this order generic divisions rest on slight
technical distinctions; and the _Campanula vidalii_ is very distinct from
any other known species. With these exceptions, most of the peculiar
Azorean species are closely allied to European plants, and are in several
cases little more than varieties of them. While therefore we may believe
that the larger part of the existing flora reached the islands since the
glacial epoch, a portion of it may be more ancient, as there is no doubt
that a majority of the species could withstand some lowering of
temperature; while in such a warm latitude and surrounded with sea, there
would always be many sunny and sheltered spots in which even tender plants
might flourish.
_Important Deduction from the Peculiarities of the Azorean Fauna and
Flora._--There is one conclusion to be drawn from the almost wholly
European character of the Azorean fauna and flora which deserves special
attention, namely, that the peopling of remote islands is not due so much
to ordinary or normal, as to extraordinary and exceptional causes. These
islands lie in the course of the south-westerly return trades and also of
the Gulf Stream, and we should therefore naturally expect that American
birds, insects, and plants would preponderate if they were {262} conveyed
by the regular winds and currents, which are both such as to prevent
European species from reaching the islands. But the violent storms to which
the Azores are liable blow from all points of the compass; and it is
evidently to these, combined with the greater proximity and more favourable
situation of the coasts of Europe and North Africa, that the presence of a
fauna and flora so decidedly European is to be traced.
The other North Atlantic Islands--Madeira, the Canaries, and the Cape de
Verdes--present analogous phenomena to those of the Azores, but with some
peculiarities dependent on their more southern position, their richer
vegetation, and perhaps their greater antiquity. These have been
sufficiently discussed in my _Geographical Distribution of Animals_ (Vol.
I. pp. 208-215); and as we are now dealing with what may be termed typical
examples of oceanic islands, for the purpose of illustrating the laws, and
solving the problems presented by the dispersal of animals, we will pass on
to other cases which have been less fully discussed in that work.
BERMUDA.
The Bermudas are a small group of low islands formed of coral, and blown
coral-sand consolidated into rock. They are situated in 32deg N. Lat.,
about 700 miles from North Carolina, and somewhat farther from the Bahama
Islands, and are thus rather more favourably placed for receiving
immigrants from America and its islands than the Azores are with respect to
Europe. There are about 100 islands and islets in all, but their total area
does not exceed fifty square miles. They are surrounded by reefs, some at a
distance of thirty miles from the main group; and the discovery of a layer
of earth with remains of cedar-trees forty-eight feet below the present
high-water mark shows that the islands have once been more extensive and
probably included the whole area now occupied by shoals and reefs.[54]
Immediately beyond these reefs, {263} however, extends a very deep ocean,
while about 450 miles distant in a south-east direction, the deepest part
of the North Atlantic is reached, where soundings of 3,825 and 3,875
fathoms have been obtained. It is clear therefore that these islands are
typically oceanic.
[Illustration: MAP OF BERMUDA AND THE AMERICAN COAST.]
NOTE.--The light tint indicates sea less than 1,000 fathoms deep.
The dark tint ,, ,, more than 1,000 fathoms deep.
The figures show the depth in fathoms.
Soundings were taken by the _Challenger_ in four {264} different directions
around Bermuda, and always showed a rapid deepening of the sea to about
2,500 fathoms. This was so remarkable, that in his reports to the
Admiralty, Captain Nares spoke of Bermuda as "a solitary peak rising
abruptly from a base only 120 miles in diameter;" and in another place as
"an isolated peak rising abruptly from a very small base." These
expressions show that Bermuda is looked upon as a typical example of an
"oceanic peak"; and on examining the series of official reports of the
_Challenger_ soundings, I can find no similar case, although some coasts,
both of continents and islands, descend more abruptly. In order to show,
therefore, what is the real character of this peak, I have drawn a section
of it on a true scale from the soundings taken in a north and south
direction where the descent is steepest. It will be seen that the slope is
on both sides very easy, being 1 in 16 on the south, and 1 in 19 on the
north. The portion nearest the islands will slope more rapidly, perhaps
reaching in places 1 in 10; but even this is not steeper than many country
roads in hilly countries, while the remainder would be a hardly perceptible
slope. Although generally very low, some parts of these islands rise to 250
feet above the sea-level, consisting of various kinds of limestone rock,
sometimes soft and friable, but often very hard and even crystalline. It
consists of beds which sometimes dip as much as 30deg, and which also show
great contortions, so that at first sight the islands appear to exhibit on
a small scale the phenomena of a disturbed Palaeozoic district. It has
however long been known that these rocks are all due to the wind, {265}
which blows up the fine calcareous sand, the product of the disintegration
of coral, shells, serpulae, and other organisms, forming sand-hills forty
and fifty feet high, which move gradually along, overwhelming the lower
tracts of land behind them. These are consolidated by the percolation of
rain-water, which dissolves some of the lime from the more porous tracts
and deposits it lower down, filling every fissure with stalagmite.
[Illustration: SECTION OF BERMUDA AND ADJACENT SEA BOTTOM.
The figures show the depth in fathoms at fifty-five miles north and
forty-six miles south of the islands respectively.]
_The Red Clay of Bermuda._--Besides the calcareous rocks there is found in
many parts of the islands a layer of red earth or clay, containing about
thirty per cent. of oxide of iron. This very closely resembles, both in
colour and chemical composition, the red clay of the ocean floor, found
widely spread in the Atlantic at depths of from 2,300 to 3,150 fathoms, and
occurring abundantly all round Bermuda. It appears, therefore, at first
sight, as if the ocean bed itself has been here raised to the surface, and
a portion of its covering of red clay preserved; and this is the view
adopted by Mr. Jones in his paper on the "Botany of Bermuda." He says,
after giving the analysis: "This analysis tends to convince us that the
deep chocolate-coloured red clay of the islands found in the lower levels,
and from high-water mark some distance into the sea, originally came from
the ocean floor, and that when by volcanic agency the Bermuda column was
raised from the depths of the sea, its summit, most probably broken in
outline, appeared above the surface covered with this red mud, which in the
course of ages has but slightly changed its composition, and yet possesses
sufficient evidence to prove its identity with that now lying contiguous to
the base of the Bermuda column." But in his _Guide to Bermuda_ Mr. Jones
tells us that this same red earth has been found, two feet thick, under
coral rock at a depth of forty-two feet below low-water mark, and that it
"rested on a bed of compact calcareous sandstone." Now it is quite certain
that this "calcareous sandstone" was never formed at the bottom of the deep
ocean 700 miles from land; and the occurrence of the red earth at different
levels upon coralline sand rock is therefore more probably due to some
process of decomposition of the rock itself, {266} or of the minute
organisms which abound in the blown sand.[55]
_Zoology of Bermuda._--As might be expected from their extreme isolation,
these islands possess no indigenous terrestrial mammalia, frogs, or
snakes.[56] There is however one lizard, which Professor Cope considers to
be distinct from any American species, and which he has named _Plestiodon
(Eumeces) longirostris_. It is said to be most nearly allied to _Eumeces
quinquelineatus_ of the south-eastern States, from which it differs in
having nearly ten more rows of scales, the tail thicker, and the muzzle
longer. In colour it is ashy brown above, greenish blue beneath, with a
white line black-margined on the sides, and it seems to be tolerably
abundant in the islands. This lizard is especially interesting as being the
only vertebrate animal which exhibits any peculiarity.
_Birds._--Notwithstanding its small size, low altitude and {267} remote
position, a great number of birds visit Bermuda annually, some in large
numbers, others only as accidental stragglers. Altogether, over 180 species
have been recorded, rather more than half being wading and swimming birds,
whose presence is not so much to be wondered at as they are great
wanderers; while about eighty-five are land birds, many of which would
hardly be supposed capable of flying so great a distance. Of the 180
species, however, about thirty have only been seen once, and a great many
more are very rare; but about twenty species of land birds are recorded as
tolerably frequent visitors, and nearly half these appear to come every
year.
There are only eleven species which are permanent residents on the
island--eight land, and three water birds, and of these one has been almost
certainly introduced. These resident birds are as follows:--
1. _Galeoscoptes carolinensis._ (The Cat bird.) Migrates along the east
coast of the United States.
2. _Sialia sialis._ (The Blue bird.) Migrates along the east coast.
3. _Vireo novaeboracensis._ (The White-eyed green Tit.) Migrates along
the east coast.
4. _Passer domesticus._ (The English Sparrow.) ? Introduced.
5. _Corvus americanus._ (The American Crow.) Common over all North
America.
6. _Cardinalis virginianus._ (The Cardinal bird.) Migrates from
Carolina southward.
7. _Chamoepelia passerina._ (The ground Dove.) Louisiana, W. Indies,
and Mexico.
8. _Ortyx virginianus._ (The American Quail.) New England to Florida.
9. _Ardea herodias._ (The Great Blue Heron.) All North America.
10. _Gailinula galeata._ (The Florida Gallinule.) Temperate and
tropical North America.
11. _Phaeeton flavirostris._ (The Tropic Bird.)
It will be seen that these are all very common North American birds, and
most of them are constant visitors from the mainland, so that however long
they may have inhabited the islands there has been no chance for them to
have acquired any distinctive characters owing to the want of isolation.
Among the most regular visitants which are not resident, are the common N.
American kingfisher (_Ceryle alcyon_), {268} the night-hawk (_Chordeiles
virginianus_), the wood wagtail (_Siurus novaeboracensis_), the
snow-bunting (_Plectrophanes nivalis_), and the wide-ranging rice-bird
(_Dolichonyx oryzivora_), all very common and widespread in North America.
_Comparison of the Bird-faunas of Bermuda and the Azores._--The bird-fauna
of Bermuda thus differs from that of the Azores, in the much smaller number
of resident species, and the presence of several regular migrants. This is
due, first, to the small area and little varied surface of these islands,
as well as to their limited flora and small supply of insects not affording
conditions suitable for the residence of many species all the year round;
and, secondly, to the peculiarity of the climate of North America, which
causes a much larger number of its birds to be migratory than in Europe.
The Northern United States and Canada, with a sunny climate, luxuriant
vegetation, and abundant insect-life during the summer, supply food and
shelter to an immense number of insectivorous and frugivorous birds; so
that during the breeding season Canada is actually richer in bird-life than
Florida. But as the severe winter comes on all these are obliged to migrate
southward, some to Carolina, Georgia, and Florida, others as far as the
West Indies, Mexico, or even to Guatemala and South America.
Every spring and autumn, therefore a vast multitude of birds, belonging to
more than a hundred distinct species, migrate northward or southward in
Eastern America. A large proportion of these pass along the Atlantic coast,
and it has been observed that many of them fly some distance out to sea,
passing straight across bays from headland to headland by the shortest
route.
Now as the time of these migrations is the season of storms, especially the
autumnal one, which nearly coincides with the hurricanes of the West Indies
and the northerly gales of the coast of America, the migrating birds are
very liable to be carried out to sea. Sometimes they may, as Mr. Jones
suggests, be carried up by local whirlwinds to a great height, where
meeting with a westerly or north-westerly gale, they are rapidly driven
sea-ward. The great majority no doubt perish, but some reach the Bermudas
{269} and form one of its most striking autumnal features. In October, Mr.
Jones tells us, the sportsman enjoys more shooting than at any other time.
The violent revolving gales, which occur almost weekly, bring numbers of
birds of many species from the American continent, the different members of
the duck tribe forming no inconsiderable portion of the whole; while the
Canada goose, and even the ponderous American swan, have been seen amidst
the migratory host. With these come also such delicate birds as the
American robin (_Turdus migratorius_), the yellow-rumped warbler
(_Dendroeca coronata_), the pine warbler (_Dendroeca pinus_), the wood
wagtail (_Siurus novaeboracensis_), the summer red bird (_Pyranga
aestiva_), the snow-bunting (_Plectrophanes nivalis_), the red-poll
(_Aegiothus linarius_), the king bird (_Tyrannus carolinensis_), and many
others. It is no doubt in consequence of this repeated immigration that
none of the Bermuda birds have acquired any special peculiarity
constituting even a distinct variety; for the few species that are resident
and breed in the islands are continually crossed by individual immigrants
of the same species from the mainland.
Four European birds also have occurred in Bermuda;--the wheatear (_Saxicola
oenanthe_), which visits Iceland and Lapland and sometimes the northern
United States; the skylark (_Alauda arvensis_), but this was probably an
imported bird or an escape from some ship; the land-rail (_Crex
pratensis_), which also wanders to Greenland and the United States; and the
common snipe (_Scolopax gallinago_), which occurs not unfrequently in
Greenland but has not yet been noticed in North America. It is however so
like the American snipe (_S. wilsoni_), that a straggler might easily be
overlooked.
Two small bats of N. American species also occasionally reach the island,
while two others from the West Indies have more rarely occurred, and these
are the only wild mammalia except rats and mice.
_Insects of Bermuda._--Insects appear to be very scarce; but it is evident
from the lists given by Mr. Jones, and more recently by Professor Heilprin,
that only the more conspicuous species have been yet collected. These {270}
comprise nineteen beetles, eleven bees and wasps, twenty-six butterflies
and moths, nine flies, and the same number of Hemiptera, Orthoptera, and
Neuroptera respectively. All appear to be common North American or West
Indian species; but until some competent entomological collector visits the
islands it is impossible to say whether there are or are not any peculiar
species.[57]
_Land Mollusca._--The land-shells of the Bermudas are somewhat more
interesting, as they appear to be the only group of animals except reptiles
in which there are any peculiar species. The following list was kindly
furnished me by Mr. Thomas Bland of New York, who has made a special study
of the terrestrial molluscs of the West Indian Islands, from which those of
the Bermudas have undoubtedly been derived. The nomenclature has been
corrected in accordance with the list given in Professor Heilprin's work on
the islands. The species which are peculiar to the islands are indicated by
italics.
LIST OF THE LAND-SHELLS OF BERMUDA.
1. Succinea fulgens. (Lea.) Also in Cuba.
2. ,, Bermudensis. (Pfeiffer.) ,, Barbadoes (?)
3. ,, margarita. (Pfr.) ,, Haiti.
4. _Poecilozonites Bermudensis._ (Pfr.) A peculiar form, which, according
to Mr. Binney, "cannot be
placed in any recognised genus."
A larger sub-fossil variety also
occurs, named _H. Nelsoni_, by
Mr. Bland, and which appears
sufficiently distinct to be
classed as another species.
5. ,, _circumfirmatas_ (Redfield.)
6. ,, _discrepans._ (Pfr.)
7. ,, _Reinianus._ (Pfr.)
8. Patula (Thysanophora) hypolepta (Shuttleworth.)
9. ,, vortex. (Pfr.) Southern Florida and West Indies.
10. Helix microdonta. (Desh.) Bahama Islands, Florida, Texas.
11. ,, appressa. (Say.) Virginia and adjacent states;
perhaps introduced into Bermuda.
{271}
12. ,, pulchella. (Muell.) Europe; very close to _H.
minuta_ (Say) of the United
States. Introduced into Bermuda
(?)
13. ,, ventricosa. (Drap.) Azores, Canary Islands, and South
Europe.
14. Bulimulus nitidulus. (Pfr.) Cuba, Haiti, &c.
15. Stenogyra octona. (Ch.) West Indies and South America.
16. Stenogyra decollata (Linn.) A South European species.
Introduced.
17. Coecilianella acicula. (Muell.) Florida, New Jersey, and Europe.
18. Pupa pellucida. (Pfr.) West Indies, and Yucatan.
19. ,, Barbadensis. (Pfr.) Barbadoes (?)
20. ,, Jamaicensis. (C. B. Ad.) Jamaica.
21. Helicina convexa. (Pfr.) Barbuda.[58]
Mr. Bland indicates only four species as certainly peculiar to Bermuda, and
another sub-fossil species; while one or two of the remainder are indicated
as doubtfully identical with those of other countries. We have thus about
one-fifth of the land-shells peculiar, while almost all the other
productions of the islands are identical with those of the adjacent
continent and islands. This corresponds, however, with what occurs
generally in islands at some distance from continents. In the Azores only
one land-bird is peculiar out of eighteen resident species; the beetles
show about one-eighth of the probably non-introduced species as peculiar;
the plants about one-twentieth; while the land-shells have about half the
species peculiar. This difference is well explained by the much greater
difficulty of transmission over wide seas, in the case of land-shells, than
of any other terrestrial organisms. It thus happens that when a species has
once been conveyed it may remain isolated for unknown ages, and has time to
become modified by local conditions unchecked by the introduction of other
individuals of the original type.
_Flora of Bermuda._--Unfortunately no good account of the plants of these
islands has yet been published. Mr. {272} Jones, in his paper "On the
Vegetation of the Bermudas" gives a list of no less than 480 species of
flowering plants; but this number includes all the culinary plants,
fruit-trees, and garden flowers, as well as all the ornamental trees and
shrubs from various parts of the world which have been introduced, mixed up
with the European and American weeds that have come with agricultural or
garden seeds, and the really indigenous plants, in one undistinguished
series. It appears too, that the late Governor, Major-General Lefroy, "has
sown and distributed throughout the islands packets of seeds from Kew,
representing no less than 600 species, principally of trees and shrubs
suited to sandy coast soils"--so that it will be more than ever difficult
in future years to distinguish the indigenous from the introduced
vegetation.
From the researches of Dr. Rein and Mr. Moseley there appear to be about
250 flowering plants in a wild state, and of these Mr. Moseley thinks less
than half are indigenous. The majority are tropical and West Indian, while
others are common to the Southern States of North America; the former class
having been largely brought by means of the Gulf Stream, the latter by the
agency of birds or by winds. Mr. Jones tells us that the currents bring
numberless objects animate and inanimate from the Carribean Sea, including
the seeds of trees, shrubs, and other plants, which are continually cast
ashore and sometimes vegetate. The soap-berry tree (_Sapindus saponaria_)
has been actually observed to originate in this way.
The only _species_ of flowering plant peculiar to Bermuda is _Carex
Bermudiana_ (Hemsley), which is said to be allied to a species found only
in St. Helena; but there are some local forms of continental species, among
which are _Sisyrinchium Bermudianum_ and a variety of _Rhus toxicodendron_.
There are, however, two ferns--an Adiantum and a Nephrodium, which are
unknown from any other locality. The juniper, which is so conspicuous a
feature of the islands, is said to be a West Indian species (_Juniperus
barbadensis_) found in Jamaica and the Bahamas, not the North American red
{273} cedar; but there seems to be still some doubt about this common
plant.
Mr. Moseley, who visited Bermuda in the _Challenger_, has well explained
the probable origin of the vegetation. The large number of West Indian
plants is no doubt due to the Gulf Stream and constant surface drift of
warm water in this direction, while others have been brought by the annual
cyclones which sweep over the intervening ocean. The great number of
American migratory birds, including large flocks of the American golden
plover, with ducks and other aquatic species, no doubt occasionally bring
seeds, either in the mud attached to their feet or in their stomachs.[59]
As these causes are either constantly in action or recur annually, it is
not surprising that almost all the species should be unchanged owing to the
frequent intercrossing of freshly-arrived specimens. If a competent
botanist were thoroughly to explore Bermuda, eliminate the species
introduced by human agency, and investigate the source from whence the
others were derived and the mode by which they had reached so remote an
island, we should obtain important information as to the dispersal of
plants, which might afford us a clue to the solution of many difficult
problems in their geographical distribution.
_Concluding Remarks._--The two groups of islands we have now been
considering furnish us with some most instructive facts as to the power of
many groups of organisms to pass over from 700 to 900 miles of open sea.
There is no doubt whatever that all the indigenous species have thus
reached these islands, and in many cases the process may be seen going on
from year to year. We find that, as regards birds, migratory habits and the
liability to be caught by violent storms are the conditions which determine
the island-population. In both islands the land-birds are almost
exclusively migrants; and in both, the non-migratory groups--wrens, tits,
creepers, and nuthatches--are absent; while the number of annual visitors
is greater in proportion as the migratory habits and prevalence of storms
afford more efficient means for their introduction. {274}
We find also, that these great distances do not prevent the immigration of
some insects of most of the orders, and especially of a considerable number
and variety of beetles; while even land-shells are fairly represented in
both islands, the large proportion of peculiar species clearly indicating
that, as we might expect, individuals of this group of organisms arrive
only at long and irregular intervals.
Plants are represented by a considerable variety of orders and genera, most
of which show some special adaptation for dispersal by wind or water, or
through the medium of birds; and there is no reason to doubt that besides
the species that have actually established themselves, many others must
have reached the islands, but were either not suited to the climate and
other physical conditions, or did not find the insects necessary to their
fertilisation, and were therefore unable to maintain themselves.
If now we consider the extreme remoteness and isolation of these islands,
their small area and comparatively recent origin, and that, notwithstanding
all these disadvantages, they have acquired a very considerable and varied
flora and fauna, we shall, I think, be convinced, that with a larger area
and greater antiquity, mere separation from a continent by many hundred
miles of sea would not prevent a country from acquiring a very luxuriant
and varied flora, and a fauna also rich and peculiar as regards all classes
except terrestrial mammals, amphibia, and some groups of reptiles. This
conclusion will be of great importance in those cases where the evidence as
to the exact origin of the fauna and flora of an island is less clear and
satisfactory than in the case of the Azores and Bermuda.
* * * * *
{275}
CHAPTER XIII
THE GALAPAGOS ISLANDS
Position and Physical Features--Absence of Indigenous Mammalia and
Amphibia--Reptiles--Birds--Insects and Land-Shells--The Keeling Islands
as Illustrating the Manner in which Oceanic Islands are Peopled--Flora
of the Galapagos--Origin of the Flora of the Galapagos--Concluding
Remarks.
The Galapagos differ in many important respects from the islands we have
examined in our last chapter, and the differences are such as to have
affected the whole character of their animal inhabitants. Like the Azores,
they are volcanic, but they are much more extensive, the islands being both
larger and more numerous; while volcanic action has been so recent that a
large portion of their surface consists of barren lava-fields. They are
considerably less distant from a continent than either the Azores or
Bermuda, being about 600 miles from the west coast of South America and a
little more than 700 from Veragua, with the small Cocos Islands
intervening; and they are situated on the equator instead of being in the
north temperate zone. They stand upon a deeply submerged bank, the 1,000
fathom line encircling all the more important islands at a few miles
distance, whence there appears to be a comparatively steep descent all
round to the average depth of that portion of the Pacific, between 2,000
and 3,000 fathoms. {276}
[Illustration: MAP OF THE GALAPAGOS AND ADJACENT COASTS OF SOUTH AMERICA.]
The light tint shows where the sea is less than 1,000 fathoms deep.
The figures show the depth in fathoms.
The whole group occupies a space of about 300 by 200 miles. It consists of
five large and twelve small islands; the largest (Albemarle Island) being
about eighty miles long and of very irregular shape, while the four next in
importance--Chatham, Indefatigable, James, and Narborough Islands, are each
about twenty-five or thirty miles {277} long, and of a rounded or elongate
form. The whole are entirely volcanic, and in the western islands there are
numerous active volcanoes. Unlike the other groups of islands we have been
considering, these are situated in a comparatively calm sea, where storms
are of rare occurrence and even strong winds almost unknown. They are
traversed by ocean currents which are strong and constant, flowing towards
the north-west from the coast of Peru; {278} and these physical conditions
have had a powerful influence on the animal and vegetable forms by which
the islands are now inhabited. The Galapagos have also, during three
centuries, been frequently visited by Europeans, and were long a favourite
resort of buccaneers and traders, who found an ample supply of food in the
large tortoises which abound there; and to these visits we may perhaps
trace the introduction of some animals whose presence it is otherwise
difficult to account for. The vegetation is generally scanty, but still
amply sufficient for the support of a considerable amount of animal life,
as shown by the cattle, horses, asses, goats, pigs, dogs, and cats, which
now run wild in some of the islands.
[Illustration: MAP OF THE GALAPAGOS.]
The light tint shows a depth of less than 1,000 fathoms.
The figures show the depth in fathoms.
_Absence of Indigenous Mammalia and Amphibia._--As in all other oceanic
islands, we find here no truly indigenous mammalia, for though there is a
mouse of the American genus Hesperomys, which differs somewhat from any
known species, we can hardly consider this to be indigenous; first, because
these creatures have been little studied in South America, and there may
yet be many undescribed species, and in the second place because even had
it been introduced by some European or native vessel, there is ample time
in two or three hundred years for the very different conditions to have
established a marked diversity in the characters of the species. This is
the more probable because there is also a true rat of the Old World genus
Mus, which is said to differ slightly from any known species; and as this
genus is not a native of the American continents we are sure that it must
have been recently introduced into the Galapagos. There can be little doubt
therefore that the islands are completely destitute of truly indigenous
mammalia; and frogs and toads, the only tropical representatives of the
Amphibia, are equally unknown.
_Reptiles._--Reptiles, however, which at first sight appear as unsuited as
mammals to pass over a wide expanse of ocean, abound in the Galapagos,
though the species are not very numerous. They consist of land-tortoises,
lizards and snakes. The tortoises consist of two peculiar species, _Testudo
microphyes_, found in most of the islands, and _T. {279} abingdonii_
recently discovered on Abingdon Island, as well as one extinct species, _T.
ephippium_, found on Indefatigable Island. These are all of very large
size, like the gigantic tortoises of the Mascarene Islands, from which,
however, they differ in structural characters; and Dr. Guenther believes
that they have been originally derived from the American continent.[60]
Considering the well known tenacity of life of these animals, and the large
number of allied forms which have aquatic or sub-aquatic habits, it is not
a very extravagant supposition that some ancestral form, carried out to sea
by a flood, was once or twice safely drifted as far as the Galapagos, and
thus originated the races which now inhabit them.
The lizards are five in number; a peculiar species of gecko,
_Phyllodactylus galapagensis_, and four species of the American family
Iguanidae. Two of these are distinct species of the genus Tropidurus, the
other two being large, and so very distinct as to be classed in peculiar
genera. One of these is aquatic and found in all the islands, swimming in
the sea at some distance from the shore and feeding on seaweed; the other
is terrestrial, and is confined to the four central islands. These last
were originally described as _Amblyrhynchus cristatus_ by Mr. Bell, and _A.
subcristatus_ by Gray; they were afterwards placed in two other genera
Trachycephalus and Oreocephalus (_see_ Brit. Mus. Catalogue of Lizards),
while in a recent paper by Dr. Steindachner, the marine species is again
classed as Amblyrhynchus, while the terrestrial form is placed in another
genus Conolophus, both genera being peculiar to the Galapagos.
How these lizards reached the islands we cannot tell. The fact that they
all belong to American genera or families indicates their derivation from
that continent, while their being all distinct species is a proof that
their arrival took place at a remote epoch, under conditions perhaps
somewhat different from any which now prevail. It is certain that animals
of this order have some means of crossing the sea not possessed by any
other land vertebrates, {280} since they are found in a considerable number
of islands which possess no mammals nor any other land reptiles; but what
those means are has not yet been positively ascertained.
It is unusual for oceanic islands to possess snakes, and it is therefore
somewhat of an anomaly that two species are found in the Galapagos. Both
are closely allied to South American forms, and one is hardly different
from a Chilian snake, so that they indicate a more recent origin than in
the case of the lizards. Snakes it is known can survive a long time at sea,
since a living boa-constrictor once reached the island of St. Vincent from
the coast of South America, a distance of two hundred miles by the shortest
route. Snakes often frequent trees, and might thus be conveyed long
distances if carried out to sea on a tree uprooted by a flood such as often
occurs in tropical climates and especially during earthquakes. To some such
accident we may perhaps attribute the presence of these creatures in the
Galapagos, and that it is a very rare one is indicated by the fact that
only two species have as yet succeeded in obtaining a footing there.
_Birds._--We now come to the birds, whose presence here may not seem so
remarkable, but which yet present features of interest not exceeded by any
other group. About seventy species of birds have now been obtained on these
islands, and of these forty-one are peculiar to them. But all the species
found elsewhere, except one, belong to the aquatic tribes or the waders
which are pre-eminently wanderers, yet even of these eight are peculiar.
The true land-birds are forty-two in number, and all but one are entirely
confined to the Galapagos; while three-fourths of them present such
peculiarities that they are classed in distinct genera. All are allied to
birds inhabiting tropical America, some very closely; while one--the common
American rice-bird which ranges over the whole northern and part of the
southern continents--is the only land-bird identical with those of the
mainland. The following is a list of these land-birds taken from Mr.
Salvin's memoir in the _Transactions of the Zoological Society_ for the
year 1876, to which are added nine species collected in 1888 and {281}
described by Mr. Ridgway in the _Proceedings of the U.S. National Museum_
(XII. p. 101) and some additional species obtained in 1889.
TURDIDAE.
1. Nesomimus trifasciatus } This and the two allied species
2. ,, melanotus } are related to a Peruvian bird
3. ,, parvulus } _Mimus longicaudus_.
4. ,, macdonaldi (Ridg.)
5. ,, personatus (Ridg.)
MNIOTILTIDAE.
6. Dendroeca aureola { Closely allied to the wide-ranging
{ _D. aestiva_.
HIRUNDINIDAE.
7. Progne concolor { Allied to _P. purpurea_ of North
{ and South America.
COEREBIDAE.
8. Certhidea olivacea } A peculiar genus allied to the
9. ,, fusca } Andean genus Conirostrum.
10. ,, cinerascens }
FRINGILLIDAE.
11. Geospiza magnirostris
12. ,, strenua
13. ,, dubia A distinct genus, but allied to the
14. ,, fortis South American genus Guiraca.
15. ,, nebulosa
16. ,, fuliginosa
17. ,, parvula
18. ,, dentirostris
19. ,, conirostris (Ridg.)
20. ,, media (Ridg.)
21. ,, difficilis (Sharpe)
22. Cactornis scandens
23. ,, assimilis
24. ,, abingdoni
25. ,, pallida A genus allied to the last.
26. ,, brevirostris (Ridg.)
27. ,, hypoleuca (Ridg.) A very peculiar genus allied to
28. Camarhynchus psittaculus Neorhynchus of the west coast
29. ,, crassirostris of Peru.
30. ,, variegatus
31. ,, prosthemelas
32. ,, habeli
33. ,, townsendi (Ridg.)
34. ,, pauper (Ridg.)
{282}
ICTERIDAE.
35. Dolichonyx oryzivorus Ranges from Canada to Paraguay.
TYRANNIDAE.
36. Pyrocephalus nanus
37. P. minimus (Ridg.) Allied to _P. rubincus_ of Ecuador.
38. Myiarchus magnirostris Allied to West Indian species.
COLUMBIDAE.
39. Zenaida galapagensis { A peculiar species of a S.
{ American genus.
FALCONIDAE.
40. Buteo galapagensis A buzzard of peculiar coloration.
STRIGIDAE.
41. Asio galapagensis } Hardly distinct from the widespread
} _A. brachyotus._
42. Strix punctatissima Allied to _S. flammea_ but quite
distinct.
We have here every gradation of difference from perfect identity with the
continental species to genera so distinct that it is difficult to determine
with what forms they are most nearly allied; and it is interesting to note
that this diversity bears a distinct relation to the probabilities of, and
facilities for, migration to the islands. The excessively abundant
rice-bird, which breeds in Canada and swarms over the whole United States,
migrating to the West Indies and South America, visiting the distant
Bermudas almost every year, and extending its range as far as Paraguay, is
the only species of land-bird which remains completely unchanged in the
Galapagos; and we may therefore conclude that some stragglers of the
migrating host reach the islands sufficiently often to keep up the purity
of the breed. Next, we have the almost cosmopolite short-eared owl (_Asio
brachyotus_), which ranges from China to Ireland, and from Greenland to the
Straits of Magellan, and of this the Galapagos bird is probably only one of
the numerous varieties. The little wood warbler (_Dendroeca aureola_) is
closely allied to a species which {283} ranges over the whole of North
America and as far south as New Grenada. It has also been occasionally met
with in Bermuda, an indication that it has considerable powers of flight
and endurance. The more distinct _species_--as the tyrant fly-catchers
(Pyrocephalus and Myiarchus), the ground-dove (Zenaida), and the buzzard
(Buteo), are all allied to non-migratory species peculiar to tropical
America, and of a more restricted range; while the distinct _genera_ are
allied to South American groups of thrushes, finches, and sugar-birds which
have usually restricted ranges, and whose habits are such as not to render
them likely to be carried out to sea. The remote ancestral forms of these
birds which, owing to some exceptional causes, reached the Galapagos, have
thus remained uninfluenced by later migrations, and have, in consequence,
been developed into a variety of distinct types adapted to the peculiar
conditions of existence under which they have been placed. Sometimes the
different species thus formed are confined to one or two of the islands
only, as the three species of Certhidea, which are divided between the
islands but do not appear ever to occur together. _Nesomimus parvulus_ is
confined to Albemarle Island, and _N. trifasciatus_ to Charles Island;
_Cactornis pallida_ to Indefatigable Island, _C. brevirostris_ to Chatham
Island, and _C. abingdoni_ to Abingdon Island.
Now all these phenomena are strictly consistent with the theory of the
peopling of the islands by accidental migrations, if we only allow them to
have existed for a sufficiently long period; and the fact that volcanic
action has ceased on many of the islands, as well as their great extent,
would certainly indicate a considerable antiquity.
The great difference presented by the birds of these islands as compared
with those of the equally remote Azores and Bermudas, is sufficiently
explained by the difference of climatal conditions. At the Galapagos there
are none of those periodic storms, gales, and hurricanes which prevail in
the North Atlantic, and which every year carry some straggling birds of
Europe or North America to the former islands; while, at the same time, the
majority of the tropical American birds are {284} nonmigratory, and thus
afford none of the opportunities presented by the countless hosts of
migrants which pass annually northward and southward along the European,
and especially along the North American coasts. It is strictly in
accordance with these different conditions that we find in one case an
almost perfect identity with, and in the other an almost equally complete
diversity from, the continental species of birds.
_Insects and Land-shells._--The other groups of land-animals add little of
importance to the facts already referred to. The insects are very scanty;
the most plentiful group, the beetles, only furnishing about forty species
belonging to thirty-two genera and nineteen families. The species are
almost all peculiar, as are some of the genera. They are mostly small and
obscure insects, allied either to American or to world-wide groups. The
Carabidae and the Heteromera are the most abundant groups, the former
furnishing six and the latter nine species.[61]
{285}
The land-shells are not abundant--about twenty in all, most of them
peculiar species, but not otherwise remarkable. The observation of Captain
Collnet, quoted by Mr. Darwin in his _Journal_, that drift-wood, bamboos,
canes, and the nuts of a palm, are often washed on the south-eastern shores
of the islands, furnishes an excellent clue to the manner in which many of
the insects and land-shells may have reached the Galapagos. Whirlwinds also
have been known to carry quantities of leaves and other vegetable _debris_
to great heights in the air, and these might be then carried away by strong
upper currents and dropped at great distances, and with them small insects
and mollusca, or their eggs. We must also remember that volcanic islands
are subject to subsidence as well as elevation; and it is quite possible
that during the long period the Galapagos have existed some islands may
have intervened between them and the coast, and have served as
stepping-stones by which the passage to them of various organisms would be
greatly facilitated. Sunken banks, the relics of such islands, are known to
exist in many parts of the ocean, and countless others, no doubt, remain
undiscovered.
_The Keeling Islands as Illustrating the Manner in which Oceanic Islands
are Peopled._--That such causes as have been here adduced are those by
which oceanic islands have been peopled, is further shown by the condition
of equally remote islands which we know are of comparatively recent origin.
Such are the Keeling or Cocos Islands in the Indian Ocean, situated about
the same distance from Sumatra as the Galapagos from South America, but
mere coral reefs, supporting abundance of cocoa-nut palms as their chief
vegetation. These islands were visited by Mr. {286} Darwin, and their
natural history carefully examined. The only mammals are rats, brought by a
wrecked vessel and said by Mr. Waterhouse to be common English rats, "but
smaller and more brightly coloured;" so that we have here an illustration
of how soon a difference of race is established under a constant and
uniform difference of conditions. There are no true land-birds, but there
are snipes and rails, both apparently common Malayan species. Reptiles are
represented by one small lizard, but no account of this is given in the
_Zoology of the Voyage of the Beagle_, and we may therefore conclude that
it was an introduced species. Of insects, careful collecting only produced
thirteen species belonging to eight distinct orders. The only beetle was a
small Elater, the Orthoptera were a Gryllus and a Blatta; and there were
two flies, two ants, and two small moths, one a Diopaea which swarms
everywhere in the eastern tropics in grassy places. All these insects were
no doubt brought either by winds, by floating timber (which reaches the
islands abundantly), or by clinging to the feathers of aquatic or wading
birds; and we only require more time to introduce a greater variety of
species, and a better soil and more varied vegetation, to enable them to
live and multiply, in order to give these islands a fauna and flora equal
to that of the Bermudas. Of wild plants there were only twenty species,
belonging to nineteen genera and to no less than sixteen natural families,
while all were common tropical shore plants.[62] These islands are thus
evidently stocked by waifs and strays brought by the winds and waves; but
their scanty vegetation is mainly due to unfavourable conditions--the
barren coral rock and sand, of which they are wholly composed, together
with exposure to sea-air, being suitable to a very limited number of
species which soon monopolise the surface. With more variety of soil and
aspect a greater variety of plants would establish themselves, and these
would favour the preservation and increase of more insects, birds, and
{287} other animals, as we find to be the case in many small and remote
islands.[63]
_Flora of the Galapagos._--The plants of these islands are so much more
numerous than the known animals, even including the insects, they have been
so carefully studied by eminent botanists, and their relations throw so
much light on the past history of the group, that no apology is needed for
giving a brief outline of the peculiarities and affinities of the flora.
The statements we shall make on this subject will be taken from the Memoir
of Sir Joseph Hooker in the _Linnaean Transactions_ for 1851, founded on
Mr. Darwin's collections, and a later paper by N. J. Andersson in the
_Linnaea_ of 1861, embodying more recent discoveries. {288}
The total number of flowering plants known at the latter date was 332, of
which 174 were peculiar to the islands, while 158 were common to other
countries.[64] Of these latter about twenty have been introduced by man,
while the remainder are all natives of some part of America, though about a
third part are species of wide range extending into both hemispheres. Of
those confined to America, forty-two are found in both the northern and
southern continents, twenty-one are confined to South America, while twenty
are found only in North America, the West Indies, or Mexico. This equality
of North American and South American species in the Galapagos is a fact of
great significance in connection with the observation of Sir Joseph Hooker
that the _peculiar_ species are allied to the plants of temperate America
or to those of the high Andes, while the non-peculiar species are mostly
such as inhabit the hotter regions of the tropics near the level of the
sea. He also observes that the seeds of this latter class of Galapagos
plants often have special means of transport, or belong to groups whose
seeds are known to stand long voyages and to possess great vitality. Mr.
Bentham also, in his elaborate account of the Compositae,[65] remarks on
the decided Central American or Mexican affinities of the Galapagos
species, so that we may consider this to be a thoroughly well-established
fact.
The most prevalent families of plants in the Galapagos are the Compositae
(40 sp.), Gramineae (32 sp.), Leguminosae (30 sp.), and Euphorbiaceae (29
sp.). Of the Compositae most of the species, except such as are common
weeds or shore plants, are peculiar, but there are only two peculiar
genera, allied to Mexican forms and not very distinct; while the genus
Lipochaeta, represented here by a single species, is only found elsewhere
in the Sandwich Islands though it has American affinities.
_Origin of the Galapagos Flora._--These facts are explained by the past
history of the American continent, its {289} separation at various epochs
by arms of the sea uniting the two oceans across what is now Central
America (the last separation being of recent date, as shown by the
considerable number of identical species of fishes on both sides of the
isthmus), and the influence of the glacial epoch in driving the temperate
American flora southward along the mountain plateaus.[66] At the time when
the two oceans were united a portion of the Gulf Stream may have been
diverted into the Pacific, giving rise to a current, some part of which
would almost certainly have reached the Galapagos, and this may have helped
to bring about that singular assemblage of West Indian and Mexican plants
now found there. And as we now believe that the duration of the last
glacial epoch in its successive phases was much longer than the time which
has elapsed since it finally passed away, while throughout the Miocene
epoch the snow-line would often be lowered during periods of high
excentricity, we are enabled to comprehend the nature of the causes which
may have led to the islands being stocked with those north tropical or
mountain types which are so characteristic a feature of that portion of the
Galapagos flora which consists of peculiar species.
On the whole, the flora agrees with the fauna in indicating a moderately
remote origin, great isolation, and changes of conditions affording
facilities for the introduction of organisms from various parts of the
American coast, and even from the West Indian Islands and Gulf of Mexico.
As in the case of the birds, the several islands differ considerably in
their native plants, many species being limited to one or two islands only,
while others extend to several. This is, of course, what might be expected
on any theory of their origin; because, even if the whole of the islands
had once been united and afterwards separated, long continued isolation
would often lead to the differentiation of species, while the varied
conditions to be found upon islands differing in size and altitude as well
as in luxuriance of vegetation, would often lead to the extinction of a
species on one island and its preservation on another. If the several
islands had been equally well {290} explored, it might be interesting to
see whether, as in the case of the Azores, the number of species diminished
in those more remote from the coast; but unfortunately our knowledge of the
productions of the various islands of the group is exceedingly unequal,
and, except in those cases in which representative species inhabit distinct
islands, we have no certainty on the subject. All the more interesting
problems in geographical distribution, however, arise from the relation of
the fauna and flora of the group as a whole to those of the surrounding
continents, and we shall therefore for the most part confine ourselves to
this aspect of the question in our discussion of the phenomena presented by
oceanic or continental islands.
_Concluding Remarks._--The Galapagos offer an instructive contrast with the
Azores, showing how a difference of conditions that might be thought
unimportant may yet produce very striking results in the forms of life.
Although the Galapagos are much nearer a continent than the Azores, the
number of species of plants common to the continent is much less in the
former case than in the latter, and this is still more prominent a
characteristic of the insect and the bird faunas. This difference has been
shown to depend, almost entirely, on the one archipelago being situated in
a stormy, the other in a calm portion of the ocean; and it demonstrates the
preponderating importance of the atmosphere as an agent in the dispersal of
birds, insects, and plants. Yet ocean-currents and surface-drifts are
undoubtedly efficient carriers of plants, and, with plants, of insects and
shells, especially in the tropics; and it is probably to this agency that
we may impute the recent introduction of a number of common Peruvian and
Chilian littoral species, and also of several West Indian types at a more
remote period when the Isthmus of Panama was submerged.
In the case of these islands we see the importance of taking account of
past conditions of sea and land and past changes of climate, in order to
explain the relations of the peculiar or endemic species of their fauna and
flora; and we may even see an indication of the effects of climatal changes
in the northern hemisphere, in the north {291} temperate or alpine
affinities of many of the plants, and even of some of the birds. The
relation between the migratory habits of the birds and the amount of
difference from continental types is strikingly accordant with the fact
that it is almost exclusively migratory birds that annually reach the
Azores and Bermuda; while the corresponding fact that the seeds of those
plants, which are common to the Galapagos and the adjacent continent, have
all--as Sir Joseph Hooker states--some special means of dispersal, is
equally intelligible. The reason why the Galapagos possess four times as
many peculiar species of plants as the Azores is clearly a result of the
less constant introduction of seeds, owing to the absence of storms; the
greater antiquity of the group, allowing more time for specific change; and
the influence of cold epochs and of alterations of sea and land, in
bringing somewhat different sets of plants at different times within the
influence of such modified winds and currents as might convey them to the
islands.
On the whole, then, we have no difficulty in explaining the probable origin
of the flora and fauna of the Galapagos, by means of the illustrative facts
and general principles already adduced.
* * * * *
{292}
CHAPTER XIV
ST. HELENA
Position and Physical Features of St. Helena--Change Effected by
European Occupation--The Insects of St.
Helena--Coleoptera--Peculiarities and Origin of the Coleoptera of St.
Helena--Land-shells of St. Helena--Absence of Fresh-water
Organisms--Native Vegetation of St. Helena--The Relations of the St.
Helena Compositae--Concluding Remarks on St. Helena.
In order to illustrate as completely as possible the peculiar phenomena of
oceanic islands, we will next examine the organic productions of St. Helena
and of the Sandwich Islands, since these combine in a higher degree than
any other spots upon the globe, extreme isolation from all more extensive
lands, with a tolerably rich fauna and flora whose peculiarities are of
surpassing interest. Both, too, have received considerable attention from
naturalists; and though much still remains to be done in the latter group,
our knowledge is sufficient to enable us to arrive at many interesting
results.
{293} [Illustration: MAP OF THE SOUTH ATLANTIC OCEAN SHOWING THE POSITION
OF ST. HELENA.]
The light tint shows depths of less than 1,000 fathoms.
The figures show depths of the sea in fathoms.
_Position and Physical Features of St. Helena._--This island is situated
nearly in the middle of the South Atlantic Ocean, being more than 1,100
miles from the coast of Africa, and 1,800 from South America. It is about
ten miles long by eight wide, and is wholly volcanic, consisting of ancient
basalts, lavas, and other volcanic products. It is very mountainous and
rugged, bounded for {294} the most part by enormous precipices, and rising
to a height of 2,700 feet above the sea-level. An ancient crater, about
four miles across, is open on the south side, and its northern rim forms
the highest and central ridge of the island. Many other hills and peaks,
however, are more than two thousand feet high, and a considerable portion
of the surface consists of a rugged plateau, having an elevation of about
fifteen hundred to two thousand feet. Everything indicates that St. Helena
is an isolated volcanic mass built up from the depths of the ocean. Mr.
Wollaston remarks: "There are the strongest reasons for believing that the
area of St. Helena was never _very_ much larger than it is at present--the
comparatively shallow sea-soundings within about a mile and a half from the
shore revealing an abruptly defined ledge, _beyond_ which no bottom is
reached at a depth of 250 fathoms; so that the original basaltic mass,
which was gradually piled up by means of successive eruptions from beneath
the ocean, would appear to have its limit definitely marked out by this
suddenly-terminating submarine cliff--the space between it and the existing
coast-line being reasonably referred to that slow process of disintegration
by which the island has been reduced, through the eroding action of the
elements, to its present dimensions." If we add to this that between the
island and the coast of Africa, in a south-easterly direction, is a
profound oceanic gulf known to reach a depth of 2,860 fathoms, or 17,160
feet, while an equally deep, or perhaps deeper, ocean, extends to the west
and south-west, we shall be satisfied that St. Helena is a true oceanic
island, and that it owes none of its peculiarities to a former union with
any continent or other distant land.
_Change Effected by European Occupation._--When first discovered, in the
year 1501, St. Helena was densely covered with a luxuriant forest
vegetation, the trees overhanging the seaward precipices and covering every
part of the surface with an evergreen mantle. This indigenous vegetation
has been almost wholly destroyed; and although an immense number of foreign
plants have been introduced, and have more or less completely established
themselves, {295} yet the general aspect of the island is now so barren and
forbidding that some persons find it difficult to believe that it was once
all green and fertile. The cause of the change is, however, very easily
explained. The rich soil formed by decomposed volcanic rock and vegetable
deposits could only be retained on the steep slopes so long as it was
protected by the vegetation to which it in great part owed its origin. When
this was destroyed, the heavy tropical rains soon washed away the soil, and
has left a vast expanse of bare rock or sterile clay. This irreparable
destruction was caused in the first place by goats, which were introduced
by the Portuguese in 1513, and increased so rapidly that in 1588, they
existed in thousands. These animals are the greatest of all foes to trees,
because they eat off the young seedlings, and thus prevent the natural
restoration of the forest. They were, however, aided by the reckless waste
of man. The East India Company took possession of the island in 1651, and
about the year 1700 it began to be seen that the forests were fast
diminishing, and required some protection. Two of the native trees, redwood
and ebony, were good for tanning, and to save trouble the bark was
wastefully stripped from the trunks only, the remainder being left to rot;
while in 1709 a large quantity of the rapidly disappearing ebony was used
to burn lime for building fortifications! By the MSS. records quoted in Mr.
Melliss' interesting volume on St. Helena,[67] it is evident that the evil
consequences of allowing the trees to be destroyed were clearly foreseen,
as the following passages show: "We find the place called the Great Wood in
a flourishing condition, full of young trees, where the hoggs (of which
there is a great abundance) do not come to root them up. But the Great Wood
is miserably lessened and destroyed within our memories, and is not near
the circuit and length it was. But we believe it does not contain now less
than fifteen hundred acres of fine woodland and good ground, but no springs
of water but what is salt or brackish, which we take to be the reason that
that part was not inhabited when the people first {296} chose out their
settlements and made plantations; but if wells could be sunk, which the
governor says he will attempt when we have more hands, we should then think
it the most pleasant and healthiest part of the island. But as to
healthiness, we don't think it will hold so if the wood that keeps the land
warm were destroyed, for then the rains, which are violent here, would
carry away the upper soil, and it being a clay marl underneath would
produce but little; as it is, we think in case it were enclosed it might be
greatly improved" ... "When once this wood is gone the island will soon be
ruined" ... "We viewed the wood's end which joins the Honourable Company's
plantation called the Hutts, but the wood is so destroyed that the
beginning of the Great Wood is now a whole mile beyond that place, and all
the soil between being washed away, that distance is now entirely barren."
(MSS. records, 1716.) In 1709 the governor reported to the Court of
Directors of the East India Company that the timber was rapidly
disappearing, and that the goats should be destroyed for the preservation
of the ebony wood, and because the island was suffering from droughts. The
reply was, "The goats are not to be destroyed, being more valuable than
ebony." Thus, through the gross ignorance of those in power, the last
opportunity of preserving the peculiar vegetation of St. Helena, and
preventing the island from becoming the comparatively rocky desert it now
is, was allowed to pass away.[68] Even in a mere {297} pecuniary point of
view the error was a fatal one, for in the next century (in 1810) another
governor reports the total destruction of the great forests by the goats,
and that in consequence the cost of importing fuel for government use was
2,729l. 7s. 8d. for a single year! About this time large numbers of
European, American, Australian, and South African plants were imported, and
many of these ran wild and increased so rapidly as to drive out and
exterminate much of the relics of the native flora; so that now English
broom gorse and brambles, willows and poplars, and some common American,
Cape, and Australian weeds, alone meet the eye of the ordinary visitor.
These, in Sir Joseph Hooker's opinion, render it absolutely impossible to
restore the native flora, which only lingers in a few of the loftiest
ridges and most inaccessible precipices, and is rarely seen except by some
exploring naturalist.
This almost total extirpation of a luxuriant and highly peculiar vegetation
must inevitably have caused the destruction of a considerable portion of
the lower animals which once existed on the island, and it is rather
singular that so much as has actually been discovered should be left to
show us the nature of the aboriginal fauna. Many naturalists have made
small collections during short visits, but we owe our present complete
knowledge of the two most interesting groups of animals, the insects, and
the land-shells, mainly to the late Mr. T. Vernon Wollaston, who, after
having thoroughly explored Madeira and the Canaries, undertook a voyage to
St. Helena for the express purpose of studying its terrestrial fauna, and
resided for six months (1875-76) in a high central position, whence the
loftiest peaks could be explored. The results of his labours are contained
in two volumes,[69] which, like all that he wrote, are models of accuracy
and research, and it is to these volumes that we are indebted for the
interesting and suggestive facts which we here lay before our readers.
{298}
_Insects--Coleoptera._--The total number of species of beetles hitherto
observed at St. Helena is 203, but of these no less than seventy-four are
common and wide-spread insects, which have certainly, in Mr. Wollaston's
opinion, been introduced by human agency. There remain 129 which are
believed to be truly aborigines, and of these all but one are found nowhere
else on the globe. But in addition to this large amount of specific
peculiarity (perhaps unequalled anywhere else in the world) the beetles of
this island are equally remarkable for their generic isolation, and for the
altogether exceptional proportion in which the great divisions of the order
are represented. The species belong to thirty-nine genera, of which no less
than twenty-five are peculiar to the island; and many of these are such
isolated forms that it is impossible to find their allies in any particular
country. Still more remarkable is the fact, that more than two-thirds of
the whole number of indigenous species are Rhyncophora or weevils, while
more than two-fifths (fifty-four species) belong to one family, the
Cossonidae. Now although the Rhyncophora are an immensely numerous group
and always form a large portion of the insect population, they nowhere else
approach such a proportion as this. For example, in Madeira they form
one-sixth of the whole of the indigenous Coleoptera, in the Azores less
than one-tenth, and in Britain one-seventh. Even more interesting is the
fact that the twenty genera to which these insects belong are every one of
them peculiar to the island, and in many cases have no near allies
elsewhere, so that we cannot but look on this group of beetles as forming
the most characteristic portion of the ancient insect fauna. Now, as the
great majority of these are wood borers, and all are closely attached to
vegetation and often to particular species of plants, we might, as Mr.
Wollaston well observes, deduce the former luxuriant vegetation of the
island from the great preponderance of this group, even had we not positive
evidence that it was at no distant epoch densely forest-clad. We will now
proceed briefly to indicate the numbers and peculiarities of each of the
families of beetles which enter into the St. Helena fauna, taking them, not
in {299} systematic order, but according to their importance in the island.
1. RHYNCOPHORA.--This great division includes the weevils and allied
groups, and, as above stated, exceeds in number of species all the other
beetles of the island. Four families are represented; the Cossonidae, with
fifteen peculiar genera comprising fifty-four species, and one minute
insect (_Stenoscelis hylastoides_) forming a peculiar genus, but which has
been found also at the Cape of Good Hope. It is therefore impossible to say
of which country it is really a native, or whether it is indigenous to
both, and dates back to the remote period when St. Helena received its
early emigrants. All the Cossonidae are found in the highest and wildest
parts of the island where the native vegetation still lingers, and many of
them are only found in the decaying stems of tree-ferns, box-wood,
arborescent Compositae, and other indigenous plants. They are all
pre-eminently peculiar and isolated, having no direct affinity to species
found in any other country. The next family, the Tanyrhynchidae, has one
peculiar genus in St. Helena, with ten species. This genus (Nesiotes) is
remotely allied to European, Australian, and Madeiran insects of the same
family: the habits of the species are similar to those of the Cossonidae.
The Trachyphloeidae are represented by a single species belonging to a
peculiar genus not very remote from a European form. The Anthribidae again
are highly peculiar. There are twenty-six species belonging to three
genera, all endemic, and so extremely peculiar that they form two new
subfamilies. One of the genera, Acarodes, is said to be allied to a
Madeiran genus.
2. GEODEPHAGA.--These are the terrestrial carnivorous beetles, very
abundant in all parts of the world, especially in the temperate regions of
the northern hemisphere. In St. Helena there are fourteen species belonging
to three genera, one of which is peculiar. This is the _Haplothorax
burchellii_, the largest beetle on the island, and now very rare. It
resembles a large black Carabus. There is also a peculiar Calosoma, very
distinct, though resembling in some respects certain African species. The
rest of the {300} Geodephaga, twelve in number, belong to the wide-spread
genus Bembidium, but they are altogether peculiar and isolated, except one,
which is of European type, and alone has wings, all the rest being
wingless.
3. HETEROMERA.--This group is represented by three peculiar genera
containing four species, with two species belonging to European genera.
They belong to the families Opatridae, Mordellidae, and Anthicidae.
4. BRACHYELYTRA.--Of this group there are six peculiar species belonging to
four European genera--Homalota, Philonthus, Xantholinus, and Oxytelus.
5. PRIOCERATA.--The families Elateridae and Anobiidae are each represented
by a peculiar species of a European genus.
6. PHYTOPHAGA.--There are only three species of this tribe, belonging to
the European genus Longitarsus.
7. LAMELLICORNIS.--Here are three species belonging to two genera. One is a
peculiar species of Trox, allied to South African forms; the other two
belong to the peculiar genus Melissius, which Mr. Wollaston considers to be
remotely allied to Australian insects.
8. PSEUDO-TRIMERA.--Here we have the fine lady-bird _Chilomenus lunata_,
also found in Africa, but apparently indigenous in St. Helena; and a
peculiar species of Euxestes, a genus only found elsewhere in Madeira.
9. TRICHOPTERYGIDAE.--These, the minutest of beetles, are represented by
one species of the European and Madeiran genus Ptinella.
10. NECROPHAGA.--One indigenous species of Cryptophaga inhabits St. Helena,
and this is said to be very closely allied to a Cape species.
_Peculiarities and Origin of the Coleoptera of St. Helena._--We see that
the great mass of the indigenous species are not only peculiar to the
island, but so isolated in their characters as to show no close affinity
with any existing insects; while a small number (about one-third of the
whole) have some relations, though often very remote, with species now
inhabiting Europe, Madeira, or South Africa. These facts clearly point to
the very great antiquity of the insect fauna of St. Helena, which has
allowed {301} time for the modification of the originally introduced
species, and their special adaptation to the conditions prevailing in this
remote island. This antiquity is also shown by the remarkable specific
modification of a few types. Thus the whole of the Cossonidae may be
referred to three types, one species only (_Hexacoptus ferrugineus_) being
allied to the European Cossonidae though forming a distinct genus; a group
of three genera and seven species remotely allied to the _Stenoscelis
hylastoides_, which occurs also at the Cape; while a group of twelve genera
with forty-six species have their only (remote) allies in a few insects
widely scattered in South Africa, New Zealand, Europe, and the Atlantic
Islands. In like manner, eleven species of Bembidium form a group by
themselves; and the Heteromera form two groups, one consisting of three
genera and species of Opatridae allied to a type found in Madeira, the
other, Anthicodes, altogether peculiar.
Now each of these types may well be descended from a single species which
originally reached the island from some other land; and the great variety
of generic and specific forms into which some of them have diverged is an
indication, and to some extent a measure, of the remoteness of their
origin. The rich insect fauna of Miocene age found in Switzerland consists
mostly of genera which still inhabit Europe, with others which now inhabit
the Cape of Good Hope or the tropics of Africa and South America; and it is
not at all improbable that the origin of the St. Helena fauna dates back to
at least as remote, and not improbably to a still earlier, epoch. But if
so, many difficulties in accounting for its origin will disappear. We know
that at that time many of the animals and plants of the tropics, of North
America, and even of Australia, inhabited Europe; while during the changes
of climate, which, as we have seen, there is good reason to believe
periodically occurred, there would be much migration from the temperate
zones towards the equator, and the reverse. If, therefore, the nearest ally
of any insular group now inhabits a particular country, we are not obliged
to suppose that it reached the island from that country, since we know that
most groups have ranged in past times over {302} wider areas than they now
inhabit. Neither are we limited to the means of transmission across the
ocean that now exist, because we know that those means have varied greatly.
During such extreme changes of conditions as are implied by glacial periods
and by warm polar climates, great alterations of winds and of
ocean-currents are inevitable, and these are, as we have already proved,
the two great agencies by which the transmission of living things to
oceanic islands has been brought about. At the present time the south-east
trade-winds blow almost constantly at St. Helena, and the ocean-currents
flow in the same direction, so that any transmission of insects by their
means must almost certainly be from South Africa. Now there is undoubtedly
a South African element in the insect-fauna, but there is no less clearly a
European, or at least a north-temperate element, and this is very difficult
to account for by causes now in action. But when we consider that this
northern element is chiefly represented by remote generic affinity, and has
therefore all the signs of great antiquity, we find a possible means of
accounting for it. We have seen that during early Tertiary times an almost
tropical climate extended far into the northern hemisphere, and a temperate
climate to the Arctic regions. But if at this time (as is not improbable)
the Antarctic regions were as much ice-clad as they are now it is certain
that an enormous change must have been produced in the winds. Instead of a
great difference of temperature between each pole and the equator, the
difference would be mainly between one hemisphere and the other, and this
might so disturb the trade winds as to bring St. Helena within the south
temperate region of storms--a position corresponding to that of the Azores
and Madeira in the North Atlantic, and thus subject it to violent gales
from all points of the compass. At this remote epoch the mountains of
equatorial Africa may have been more extensive than they are now, and may
have served as intermediate stations by which some northern insects may
have migrated to the southern hemisphere.
We must remember also that these peculiar forms are said to be northern
only because their nearest allies are {303} now found in the North Atlantic
islands and Southern Europe; but it is not at all improbable that they are
really widespread Miocene types, which have been preserved mainly in
favourable insular stations. They may therefore have originally reached St.
Helena from Southern Africa, or from some of the Atlantic islands, and may
have been conveyed by oceanic currents as well as by winds.[70] This is the
more probable, as a large proportion of the St. Helena beetles live even in
the perfect state within the stems of plants or trunks of trees, while the
eggs and larvae of a still larger number are likely to inhabit similar
stations. Drift-wood might therefore be one of the most important agencies
by which these insects reached the island.
Let us now see how far the distribution of other groups support the
conclusions derived from a consideration of the beetles. The Hemiptera have
been studied by Dr. F. Buchanan White, and though far less known than the
beetles, indicate somewhat similar relations. Eight out of twenty-one
genera are peculiar, and the thirteen other genera are for the most part
widely distributed, while one of the peculiar genera is of African type.
The other orders of insects have not been collected or studied with {304}
sufficient care to make it worth while to refer to them in detail; but the
land-shells have been carefully collected and minutely described by Mr.
Wollaston himself, and it is interesting to see how far they agree with the
insects in their peculiarities and affinities.
_Land-shells of St. Helena._--The total number of species is only
twenty-nine, of which seven are common in Europe or the other Atlantic
islands, and are no doubt recent introductions. Two others, though
described as distinct, are so closely allied to European forms, that Mr.
Wollaston thinks they have probably been introduced and have become
slightly modified by new conditions of life; so that there remain exactly
twenty species which may be considered truly indigenous. No less than
thirteen of these, however, appear to be extinct, being now only found on
the surface of the ground or in the surface soil in places where the native
forests have been destroyed and the land not cultivated. These twenty
peculiar species belong to the following genera: Hyalina (3 sp.), Patula (4
sp.), Bulimus (7 sp.), Subulina (3 sp.), Succinea (3 sp.); of which, one
species of Hyalina, three of Patula, all the Bulimi, and two of Subulina
are extinct. The three Hyalinas are allied to European species, but all the
rest appear to be highly peculiar, and to have no near allies with the
species of any other country. Two of the Bulimi (_B. auris vulpinae_ and
_B. darwinianus_) are said to somewhat resemble Brazilian, New Zealand, and
Solomon Island forms, while neither Bulimus nor Succinea occur at all in
the Madeira group.
Omitting the species that have probably been introduced by human agency, we
have here indications of a somewhat recent immigration of European types
which may perhaps be referred to the glacial period; and a much more
ancient immigration from unknown lands, which must certainly date back to
Miocene, if not to Eocene, times.
_Absence of Fresh-water Organisms._--A singular phenomenon is the total
absence of indigenous aquatic forms of life in St. Helena. Not a single
water-beetle or fresh-water shell has been discovered; neither do there
seem to be any water-plants in the streams, except the common {305}
water-cress, one or two species of Cyperus, and the Australian _Isapis
prolifera_. The same absence of fresh-water shells characterises the
Azores, where, however, there is one indigenous water-beetle. In the
Sandwich Islands also recent observations refer to the absence of
water-beetles, though here there are a few fresh-water shells. It would
appear therefore that the wide distribution of the same generic and
specific forms which so generally characterises fresh-water organisms, and
which has been so well illustrated by Mr. Darwin, has its limits in the
_very remote_ oceanic islands, owing to causes of which we are at present
ignorant.
The other classes of animals in St. Helena need occupy us little. There are
no indigenous mammals, reptiles, fresh-water fishes or true land-birds; but
there is one species of wader--a small plover (_Aegialitis
sanctae-helenae_)--very closely allied to a species found in South Africa,
but presenting certain differences which entitle it to the rank of a
peculiar species. The plants, however, are of especial interest from a
geographical point of view, and we must devote a few pages to their
consideration as supplementing the scanty materials afforded by the animal
life, thus enabling us better to understand the biological relations and
probable history of the island.
_Native Vegetation of St. Helena._--Plants have certainly more varied and
more effectual means of passing over wide tracts of ocean than any kinds of
animals. Their seeds are often so minute, of such small specific gravity,
or so furnished with downy or winged appendages, as to be carried by the
wind for enormous distances. The bristles or hooked spines of many small
fruits cause them to become easily attached to the feathers of aquatic
birds, and they may thus be conveyed for thousands of miles by these
pre-eminent wanderers; while many seeds are so protected by hard outer
coats and dense inner albumen, that months of exposure to salt water does
not prevent them from germinating, as proved by the West Indian seeds that
reach the Azores or even the west coast of Scotland, and, what is more to
the point, by the fact stated by Mr. Melliss, that large seeds which have
floated from {306} Madagascar or Mauritius round the Cape of Good Hope,
have been thrown on the shores of St. Helena and have then sometimes
germinated!